1. Field of the Invention
The present invention relates to an image forming apparatus including an image heating device that heats an image formed on a recording material using toner containing a toner-parting agent.
2. Description of the Related Art
Conventionally, in an image forming apparatus, such as a printer and a copying machine, which uses an electrophotographic process, an image formed on a recording material by toner formed of a binder resin is heated by an image heating device. As the image heating device, a fixing device is often used, which uses a heat-roller method in which a recording material is sandwiched, conveyed, and heated by a heating roller (fixing roller) kept at a predetermined temperature and a pressure roller that has an elastic layer and presses the heating roller.
In particular, an image forming apparatus that forms a color image generally uses color toner of cyan, magenta, yellow, and the like, and generally uses nonmagnetic toner formed of a material having sharp-melting properties with a low melting point and a low melt viscosity compared with monochrome toner.
When fixing such a color image, color mixing properties are required for the toner, and toner having high sharp-melting properties and excellent melting characteristics is preferable. However, toner having high sharp-melting properties tends to be offset if toner-parting properties from the surface of a fixing roller are not sufficient when the toner is fixed.
To improve the toner-parting properties, a fixing method of an oil-coating method, in which silicon oil or the like is coated on the surface of the fixing roller, is conventionally used. However, there is an issue that the apparatus becomes complicated because an oil-coating device needs to be provided. Further, there is an issue that an oil-streak-gloss variation occurs due to the oil-coating variation.
Therefore, an oilless fixing method which uses oilless toner in which a toner-parting agent, such as paraffin wax, polyethylene wax, and silicon wax, is added to toner and a fixing roller, having a fluororesin surface layer, has been developed.
However, it is known that, in an oilless fixing device using the above-described oilless toner containing wax, the gloss of an image changes depending on the cooling state of the fixed toner and the wax. This is because the degree of crystallinity, the degree of transparency, and the surface properties change depending on the solidification state of the wax. The techniques described below related to a gloss-variation issue generated by a variation of the solidification state of the wax are known.
For example, in the invention discussed in Japanese Patent Laid-Open No. 2005-266079, it is prevented that diffuse reflection and incident light absorption occur due to wax components deposited on an image surface of the recording material, and image gloss degradation as well as muddy color occur. Specifically, in the invention, the above-described wax is removed after fixing.
In the invention discussed in Japanese Patent Laid-Open No. 2006-091146, it is prevented that a toner image on a recording material discharged from a fixing device comes into contact with a conveying roll and the like, to prevent wax on the surface of the toner image from rapidly cooled below the melting-point temperature, thereby preventing a roller mark from occurring when the wax crystallizes. Specifically, a heating roll of the fixing device has many small holes and the melted wax is absorbed.
In the invention discussed in Japanese Patent Laid-Open No. 2006-003404, it is prevented that a contact scar, which is a so-called roller mark (roll mark), occurs in a toner image when a member such as a roller comes into contact with the fixed toner image. Specifically, the recording material is conveyed in a state in which no member comes into contact with the toner image that has passed through a fixing unit until the temperature of the toner image heated by the fixing unit falls below the melting-point temperature of the wax component.
However, the image forming apparatuses described above have issues described below.
In Japanese Patent Laid-Open No. 2005-266079, wax is removed after fixing. However, it is difficult to evenly and uniformly remove wax on the image surface after fixing, and a gloss variation due to removal variation occurs after long-term use.
In Japanese Patent Laid-Open No. 2006-091146, the heating roll has many small holes and the melted wax is absorbed. However, in the same manner as described above, it is difficult to evenly and uniformly remove wax by the absorption, and a gloss variation due to removal variation occurs after long-term use.
In Japanese Patent Laid-Open No. 2006-003404, a conveying-system configuration is used in which a conveying roller does not come into contact with the surface of the toner image until the temperature falls below the melting-point temperature of the wax. However, in a case of a high-speed machine having a high conveying speed, there is an issue that the length of a conveying portion, in which the conveying roller does not come into contact with the surface of the toner image, is very long and the size of an apparatus main body increases significantly. Even when the wax on the surface of the toner image solidifies, if the wax inside the toner image does not solidify, only the surface of the toner layer is hardened, and the entire toner layer including a lower layer does not have rigidity. In such a state, when the surface of the toner layer comes into contact with a member such as a conveying roller, a conveying guide, and a separation claw, not only the surface of the toner layer, but also the lower layer are deformed, so that a gloss variation occurs.
On the other hand, when the wax of the toner solidifies, if the wax solidifies from the lower layer of the toner layer and thereafter a portion near the surface of the toner layer solidifies, uniform surface properties can be obtained and the gloss of the image increases.